example_code.move

 module hello_blockchain::message {
            use std::error;
            use std::signer;
            use std::string;
            use aptos_framework::account;
            use aptos_framework::event;

        //:!:>resource
            struct MessageHolder has key {
                message: string::String,
                message_change_events: event::EventHandle<MessageChangeEvent>,
            }
        //<:!:resource

            struct MessageChangeEvent has drop, store {
                from_message: string::String,
                to_message: string::String,
            }

            /// There is no message present
            const ENO_MESSAGE: u64 = 0;

            #[view]
            public fun get_message(addr: address): string::String acquires MessageHolder {
                assert!(exists<MessageHolder>(addr), error::not_found(ENO_MESSAGE));
                borrow_global<MessageHolder>(addr).message
            }

            public entry fun set_message(account: signer, message: string::String)
            acquires MessageHolder {
                let account_addr = signer::address_of(&account);
                if (!exists<MessageHolder>(account_addr)) {
                    move_to(&account, MessageHolder {
                        message,
                        message_change_events: account::new_event_handle<MessageChangeEvent>(&account),
                    })
                } else {
                    let old_message_holder = borrow_global_mut<MessageHolder>(account_addr);
                    let from_message = old_message_holder.message;
                    event::emit_event(&mut old_message_holder.message_change_events, MessageChangeEvent {
                        from_message,
                        to_message: copy message,
                    });
                    old_message_holder.message = message;
                }
            }

            #[test(account = @0x1)]
            public entry fun sender_can_set_message(account: signer) acquires MessageHolder {
                let addr = signer::address_of(&account);
                aptos_framework::account::create_account_for_test(addr);
                set_message(account,  string::utf8(b"Hello, Blockchain"));

                assert!(
                get_message(addr) == string::utf8(b"Hello, Blockchain"),
                ENO_MESSAGE
                );
            }
        }